Method of vaporizing liquids by microwave heating

ABSTRACT

This invention is directed to a method for vaporizing a liquid by slowly evaporating the liquid from a liquid phase to a vapor phase below the boiling point of the liquid; and applying an effective amount of microwave power to maintain the slow evaporation of the liquid to produce a purified liquid.

FIELD OF THE INVENTION

This invention is generally related to a method for purifying liquids.More specifically, this invention is related to a method for purifyingliquid using microwave energy during the vaporizing process to produceultra high purity gases.

BACKGROUND OF THE INVENTION

Process gases for use in semiconductor manufacturing facilities aregenerally supplied through cylinders. In order to meet the increasingdemand for high flow rate product and ultra-high purity requirements forthese gases, gas producers often use an ultra-high purity bulk vaporizerdelivery system to treat polar liquefied compressed gases. In contrast,the proposed invention is an on-site system that uses a microwave sourceof power to increase and to control the evaporation rate of aqueouspolar liquids (such as ammonia). The on-site system is able to providepurification, short response time, and accurately controllablevaporization at very high flow rates (exceeding 1000 liters/min in largevolumes for the semiconductor industry). To sustain the vaporization ofthe polar liquid compound, power must be added to the system to replacethe heat that is carried off with the gaseous flow to the customer usepoint. If this is not done, then, in the case of ammonia, the pressureand the temperature at the gas/liquid interface will drop and the polarliquid's vaporization rate will decrease until the gas-liquid pooleventually sub-cools and the vapor flow decreases to a negligible level.

The on-site vaporization system delivers power though a wave-guide and aquartz window. There is no solid source of contamination because thereis no physical contact between the energy source and the polar liquid.As a result, the on-site vaporizer system reduces the amount ofimpurities that are introduced into the vapor phase, thus adding apurification step.

The impurities that will be excluded from the vapor phase and remain inthe liquid pool as a result of liquid evaporation includes Group I,Group II and Group III metals, as well as oxides, carbonates, hydridesand halides of these elements. These impurities, for a conventionalammonia vaporizer involving vaporization using wall heating, come from anumber of sources including friction from ammonia valves actuation,thermal expansion and contraction of liquid containers, pressure stretchor expansion on vessel container openings during filling, etc. Moistureis another impurity. Based on vapor-liquid equilibrium data, theconcentration of moisture in the liquid phase is approximately 2 to 3orders of magnitude greater than its concentration in the vapor phase.Careful handling in evaporating the liquid phase to the vapor phase canreduce moisture and other non-volatile residues (NVR) by orders ofmagnitude.

As illustrated in Table I, the semiconductor industry requires thatcritical impurities be removed from ammonia or other polar fluid. Thecritical impurities to be removed include moisture and NVRs such as oilsand hydrocarbons. Generally, liquid ammonia feed contains 3,000 ppbmoisture and 2,000 ppb NVR and oil. For semiconductor purposes, theinvention's objective is to achieve less than 100 ppb moisture and 100ppb NVR impurity level in ammonia or polar product fluid. Anotherobjective is to achieve less than 5.0 ppb metals impurity level inammonia or polar product fluid, although, we expect less than 1 pptmetal in the vapor product.

TABLE I Critical Impurity Levels Before and After Purification ImpurityLevel in UHP Impurity Level Gaseous Ammonia Impurity in Feed AcceptablePreferred Moisture 3,000 ppb <100 ppb   10 ppb NVR 2,000 ppb <100 ppb<10 ppb Metals    5 ppb  <5 ppb  <3 ppb

A common prior art practice for delivering purified gas is through thevaporizer approach, which uses electrical-resistance heating. Theprocess withdraws polar liquid from a tank and heats it with a heatsource from either an internal heater or external band heaters toprovide heat in excess of available ambient heat. The vapor then passesthrough a heat exchanger located within the pool of polar liquid topromote further vaporization it the tank. Conduction and convective heattransfer to the bulk liquid from a line source increases the systemresponse time. The use of immersion heaters or shell and tube heatexchangers promotes nucleus boiling, releasing vapor bubbles from thenucleation sites. Since the nucleation sites on the heating surface aresubstantially hotter than the bulk liquid vaporization of impurities,such as water, takes place. As temperature gradients and nucleationsites are generated, the agitation level in the bulk liquid isincreased. Agitation also increases the mobility of NVR, increasing thechances of allowing passage into the vapor phase. As mentioned, thecurrent invention avoids these problems because there is no physicalcontact between the power source and the liquid. With no immersionheater or heating through container walls, there are no surface hotspots, nor is there any appreciable attendant liquid agitation. Releaseand carry over of contaminating impurities is, thus, greatly reducedusing the current invention.

U.S. Pat. No. 4,671,952 discloses a process and apparatus for generatingsulfur dioxide vapor from contaminated liquid sulfur dioxide. Theprocess uses contaminated liquid sulfur dioxide and subjects it tomicrowave energy at a frequency of 915, 2450, 5850 or 18000 Mhz for asufficient period of time to produce sulfur dioxide vapor, collectingthe vapor and removing the remaining contaminated liquid sulfur dioxide,The vapor pressure of the sulfur dioxide is 34.4 psig at 70° F. andpurity of sulfur dioxide achieved is 98.99%. This patent does not teachor suggest the concept of vaporizing the liquid from the discretepenetration depth from the exposed liquid/vapor interface at the top ofthe liquid mass. Nor does it teach that this process will produceultra-high purity vapor product. Certainly, there is no teaching orsuggestion for segregating the heated liquid from the bulk liquid.

U.S. Pat. No. 4,285,774 discloses an apparatus that continuouslyproduces concentrated alcohol from beer. A plurality of concentratorcells and a plurality of salvage cells are arranged in a line inside-by-side relation. Beer is supplied to the first upstreamconcentrator cell through a supply conduit. The beer then flows throughpassages between adjacent cells in response to the volume of beerreaching a predetermined level in the adjacent upstream cell. Amicrowave ignition bulb is positioned in each cell to heat the beer andboil or vaporize the alcohol content. The gaseous alcohol seriallybubbles through a fluid passage from each concentrator cell to the nextadjacent upstream cell until the gaseous alcohol reaches the firstconcentrator cell where the gaseous alcohol is concentrated andcondensed in a column to a liquid solution containing approximately 95%alcohol and approximately 5% water. The alcohol obtained from thedilute, substantially spent beer in the salvage cells is collected andreturned to the supply conduit for recycle. This invention purposelyboils and vaporizes the alcohol in the liquid beer feed. To contrast,the instant invention avoids vaporizing the bulk of the liquid feed inorder to increase the purity level of the gaseous product.

U.S. Pat. No. 5,882,416 discloses a liquid delivery system fordelivering a liquid reagent in vaporized form to a chemical vapordeposition reactor. The reactor is arranged in a vapor-receivingrelationship to the liquid delivery system. The liquid delivery systemincludes an elongated vaporization fluid flow passage defined by alongitudinal axis and bounded by an enclosing wall. Vaporization isachieved using a heating element contained within the fluid flow passagetransverse to the longitudinal axis for heating the fluid tovaporization. The vaporized liquid is then carried to a chemical vapordeposition reactor.

U.S. Pat. No. 5,846,386 discloses an on-site vaporizer that drawsammonia vapor from a liquid ammonia reservoir. The ammonia vapor thenpasses through a microfiltration filter, and is then scrubbed usinghigh-pH purified water. Commercial grade ammonia converts tosufficiently high-purity ammonia without the need for conventionalcolumn distillation. Liquid ammonia is stored in a reservoir. Anexternal immersion heat source generates vapor from the liquid ammoniasupply reservoir. Such vaporization constitutes a single stagedistillation, leaving certain solid impurities and high-boilingimpurities behind in the liquid phase. The ammonia vapor drawn from thevapor space in the reservoir passes through a microfilter. A pressureregulator controls the flow of the filtered vapor and directs it to ascrubbing column/circulation pump combination and then to either adistillation column, a deionized water dissolving unit for purifiedliquid product point of use, or to transfer lines for gaseous point ofuse. The vapor headspace of the reservoir controls the flow rate. Acirculation pump is employed in the vaporizer system, which can be asource of metallic impurities.

U.S. Pat. No. 5,523,652 discloses using microwave energy in a dielectricplasma chamber, a pair of vaporizers, a microwave tuning andtransmission assembly and a magnetic field generating assembly. Thechamber defines an interior region in which a source gas is routed andionized to form plasma. The microwave tuning and transmission assemblyfeeds microwave energy to the chamber in the TE₁₀ (transverse electric)mode.

None of the prior art is believed to teach or suggest using microwavepower to control vaporizing a polar liquid within the thermallysegregated penetration depth of the liquid pool and produce ultra highpurity gases.

As used herein, “penetration depth” (P_(D)) means the depth of theliquid that is actually heated by the microwave power in this invention.

As used herein, “liquid depth” (L_(D)) means the bulk liquid that isessentially unaffected by the microwave power.

As used herein, “flux” means the power (P) per unit of exposed fluidarea (A_(S)) exposed to the microwave energy (expressed in Watts/ft²).

As used herein, “freeboard” means the exposed fluid area exposed to themicrowave energy.

As used herein, “ripple” or “disturbance” (R_(D)) means the layer at thetop of the penetration depth that has enough movement to start enteringinto the vapor phase, measured from the top of the crest to the bottomof the trough.

As used herein, “superheat” means the temperature above the boilingpoint of the liquid and reflects the thermal driving force (ΔT) requiredto achieve boiling condition for the liquid. As an illustration, if theboiling point is 25 degrees and the liquid is at 29 degrees, then theamount of superheat is 4 degrees.

As used herein, “agitation point” means the point where agitation of theliquid being vaporized begins. It is above the boiling point, theR_(D)<0.5″ and preferably R_(D)<0.1″.

SUMMARY OF THE INVENTION

An aspect of this invention is directed to a method for separating apolar liquid from a liquid and gaseous mixture comprising a) introducingan effective amount of microwave energy at a depth of up to about 30 mminto the liquid below the surface of the liquid; b) controlling themicrowave power that is introduced into the liquid to maintain theliquid at a condition substantially below the agitation point of theliquid; c) evaporating the liquid in the presence of the microwave powerwhile maintaining the temperature close to the boiling point of theliquid to form purified vapors; and d) capturing the evaporated purifiedvapors.

Another aspect of this invention is directed to a method for separatinga polar liquid from a liquid and gaseous mixture comprising introducingmicrowave power at a depth of up to about 30 mm into the liquid belowthe surface of the liquid to reach the liquid's boiling point;controlling the microwave power to near the boiling point of the liquid;evaporating the liquid in the presence of the microwave energy whilemaintaining the temperature substantially below the agitation point ofthe liquid to form purified vapors; and capturing the evaporatingpurified vapors.

The microwave power is introduced into the liquid to a temperature inthe range from about 5° C. below the boiling point of the liquid toabout 10° C. above the boiling point by controlling the vapor pressureof the liquid, with the preferred condition being close to the boilingpoint of the liquid.

The resulting purity of the purified gas is less than 100 ppb ofimpurities.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of preferred embodiments and theaccompanying drawings.

FIG. 1 is a sketch showing the different temperature regions in thevaporizer, 1. The bulk fluid region, 2, is essentially unheated, whilethe heated region depicted by the penetration depth, 3, is operatingcloser to the boiling temperature of the fluid. Within the penetrationdepth there is a temperature gradient. Toward the top of the penetrationdepth, the fluid is at a higher temperature than at the bottom of thepenetration depth, 5. Microwave energy used, to heat the fluid, isintroduced into the vaporizer from the top control region, 6, of thevaporizer vessel.

FIG. 1 a depicts the ripple or disturbance effect.

FIG. 2 illustrates the relationship between temperature, T_(PD), and itsvapor-phase flow rate in the penetration depth of the polar liquid, aswell as the relationship between the temperature and the gas-phaseimpurity levels. As temperature increases, flow rate increases, alongwith an increase in impurity level.

FIG. 3 illustrates the liquid-vapor equilibrium for ammonia, along withthe design conditions for the invention.

FIG. 4 provides a schematic diagram of the storage, evaporation,control, and delivery method in this invention.

FIG. 5 is the vapor-liquid equilibrium curve for ammonia.

FIG. 6 is a diagram illustrating flow as a function of flux. Flux is theinput of watts per heated area.

FIG. 7 shows that impurity decreases as a function of time. This figuresuggests that after about 1 hour, the impurity stabilizes and reaches abottom plateau.

DETAILED DESCRIPTION OF THE INVENTION

This invention uses impure (99.9995% pure or 5 ppm) polar liquid (i.e.,ammonia) and adds heat thereto to vaporize the polar liquid whileobtaining higher purity (99.99994% or 0.6 ppm or 600 ppb) in the vaporphase. Microwave radiation is used to maintain the temperature of thepolar liquid pool within the penetration depth near the boiling point,T_(B), of the polar liquid at that pressure, FIG. 2. As can be seen fromFIG. 2, as temperature of the penetration depth increases the flow rateincreases. However, there is also a concurrent increase in impuritylevel in the vapor phase. Above the agitation point, T_(A), the vaporphase impurity level increases rapidly. The objective of producing goodvaporization flow rate while maintaining relatively low impurity levelsin the vapor phase could be achieved by operating the vaporizer withtemperature of the liquid in the penetration depth at near T_(B), butbelow T_(A). By operating the vaporizer at temperature essentially belowthe agitation point, the attendant agitation of the polar liquid isessentially avoided. However, it is preferable to apply sufficientmicrowave energy to reach the boiling temperature of the polar liquid,and then to adjust the microwave power to achieve a liquid temperaturebelow the agitation point of the polar liquid, but enough microwaveenergy to vaporize the polar liquid at the defined high flow rate.

The vaporization method in this invention produces higher purity vaporammonia by evaporating impure using microwave energy. The impurity levelof the resulting vapor ammonia is less than about 10 ppm, preferablyless than about 5 ppm, and most preferably less than about 1 ppm.

Generally, the vapor impurity level in the resulting vapor ammonia isless than about 3 ppm, preferably less than about 1 ppm, and mostpreferably less than about 0.2 ppm.

In the practice of this invention, vaporized product is withdrawn fromthe top of the evaporator, the vapor having a much lower concentrationof impurities than that in the liquid phase. The microwave source causesthe liquid to evaporate.

Microwave heating of polar liquid is based on the premise that thepositive and negative charges are not coincident in space. Consequently,the molecules have a tendency of orienting themselves in response to theelectric field. However, the electric field in the microwave region ofthe electromagnetic spectrum oscillates at a rate that far exceeds themovement of the molecules. This creates an internal resisting force,which interacts with the electric field and generates heat.

A number of microwave frequencies for this type of viscous heating mayapply. However, a higher frequency, which lowers the penetration depth,is preferred. A smaller penetration depth localizes the energy andallows for more efficient utilization of energy and less disruption ofthe volume away from the evaporation point. Further, at 2,450 MHz is thepreferred frequency due to the ease of the availability of the equipmentand the wave-guide size is small and allows for ease of handling.Nonetheless, other frequencies up to 18,000 MHz can be used. The maincriteria is to ensure that just enough internal friction is generated bythe electric field of the electromagnetic wave to increase the vaporpressure of the saturated liquid to sustain a required flow rate to thecustomer. In this invention, 915 MHz is the preferred frequency, 18,000MHz is the more preferred frequency and 2,450 MHz is the most preferredfrequency.

It is believed that this use of microwave energy to purify polar liquidmaintains its vapor phase purity at high levels for two reasons. First,minimizing of agitation, which would occur in the system if superheat iskept to a minimum, keeps the impurities in the bulk fluid region towardsthe bottom of the liquid pool where vaporization is not occurring.Second, with essentially no hot spots, moisture, organic oils, andnon-volatile residues will remain in the liquid phase of the polarliquid.

From the available vapor-liquid equilibrium data for themoisture-ammonia system and from available data on moistureconcentrations in the liquid and gas phases, it is well known that theconcentration of moisture in the liquid phase is at least 2 to 3 ordersof magnitude greater than its concentration in the vapor phase atambient conditions. A similar or greater concentration differencebetween phases of trace oils and other NVRs has been observed.

This invention provides for a careful vapor phase transfill bysubstantially preventing the system from the boiling conditions. Bymaintaining the temperature near the boiling point of the polar liquid,moisture, organic oils and non-volatile residues remain in the liquidphase without transferring into the vapor phase. However, if the polarliquid is allowed to boil with large “superheat” for an extended period,some of the moisture, organic oils and non-volatile residues would beable to transfer (in large concentrations) to the vapor phase byvaporization and agitation, thereby defeating the objective of thisinvention.

Careful vapor phase transfill can reduce moisture content by two ordersof magnitude, i.e., from 100 ppm to less than 10-100 ppb. Higherpurification is achieved when the flow rate of the transfill is carriedout slowly enough to prevent rapid boiling and high agitation of theliquid ammonia. If significant agitated boiling occurs, the desiredpurification may not be accomplished, as more of the impurities(moisture) in the bulk liquid get transported in the P_(D) and enter thegas phase. However, at a low flow, vapor phase transfill will allow thesystem to maintain the favorable vapor-liquid equilibrium moisturedistribution and produce the desired two order of magnitude moistureconcentration reduction. A significant reduction in vapor phase oil andNVR concentration will also occur at the same time. Ultimately, theinvention is able to produce vapor ammonia with less than 1000 parts perbillion, more preferably less than 10 ppb, of impurity levels.

This invention uses microwave energy to provide the heat ofvaporization, while substantially maintaining the vapor-liquidequilibrium in the vessel near its boiling point.

Boiling occurs as the evaporated polar liquid is drawn off the top ofthe vapor fill. As the evaporated liquid is drawn off the top, the vaporpressure decreases. Also, as the polar vapor is drawn off the top of thevapor fill, the temperature of the liquid drops causing the liquid tosubcool. The subcooled liquid will spot boil at the reduced vaporpressure. Point 3′ of FIG. 3 helps illustrate this point. To preventspot boiling, the microwave power is increased to provide heat to thesubcooled liquid resulting in increased vapor flow and rise in the vaporpressure. This, in turn, reestablishes the vapor-liquid/pressuretemperature equilibrium. The equilibrium of the vessel is at atemperature close to the boiling point for that pressure. Byreestablishing the appropriate pressure and temperature equilibrium, theinvention operates to prevent spot boiling of the polar liquid. Also,this method of operation restores the desired evaporation rate, whilemaintaining the purity of vaporized ammonia.

The benefits for using microwave energy as a source of heat includes: 1)rapid response to replace the heat of vaporization energy so as to avoidboiling ammonia in liquid pool; 2) absence of particulate contamination;and 3) efficient energy use. Microwave energy sources are very efficientfor heating polar substances, where the positive and negative chargecenters are separated in space even though the net charge on themolecule is zero. Thus, in polar substances like water or in ammonia,the charge separation enables coupling of the molecules to the energy,thus resulting in heating.

Microwave energy is delivered through wave-guide 7 and quartz window 8as shown in FIG. 1. There is little or no contamination because theenergy is delivered without generating nucleation sites for boilingammonia or other impurities, such as water.

Microwave energy, when delivered into a tank holding a liquid, willpenetrate the liquid to a depth depending on the permittivity,permeability, and microwave mode of operation resulting in heating onlythe region of the liquid proximate the microwave input to the tank. Theliquid depth of penetration decreases as the microwave frequencyincreases. As the surface recedes by evaporation, further layers getheated. Since the heating element is the liquid itself, when the poweris turned off, heating ceases instantly, resulting in a “rapidresponse”.

Volumetric heating produces high efficiency because it does not dependon surface conduction and convention to any great extent. Further, thedisturbance (penetration depth) is estimated at less than about 0.25″,preferably less than about 0.1″. Almost all of the gas reaching thefreeboard surface, 9, (FIG. 1) in the tank by evaporation will,accordingly, come from the heat penetration depth ranging from 2.6 mm to30 mm in our case. The stated freeboard surface is the actual interfacebetween the liquid and vapor phases.

In order to satisfy the need for safe and cost effective bulk source anddelivery system, the present invention provides a method and apparatusfor delivering ultra high purity polar process gases. The deliverysystem is an evaporation system capable of sustaining a high flow rate.The evaporation system contains a large quantity of polar saturatedliquid chemical product where the chemical has at least a vapor phaseand a liquid phase.

One method of operation is to run the process in a batch mode. The polarliquid is allowed to decrease to a certain level, and then turn themicrowave off, while the vessel will subsequently be refilled with freshpolar liquid. It is important that the level of bulk liquid does notbecome too low. The bulk liquid height, L_(D), should never be smallerthan the penetration depth height, P_(D). Otherwise, the impurities inthe bulk liquid region will become too concentrated and may be able totransfer to the vaporized ammonia product.

In addition to batch mode operation, another method of operation isthrough continuous addition of polar liquid from cylinder 1 (FIG. 1)while the microwave energy is applied.

FIG. 1 depicts the general operation of this invention. Impure feedliquid ammonia is fed to the vaporizer (not shown). The microwave energysource supplies power to the vaporizer system through wave guide 7. Themicrowave power heats a penetration depth of up to about 30 mm of thetop of the liquid ammonia, 3. Below this penetration depth is bulkliquid 2, which is relatively unheated. Relatively unheated means thatthe bulk liquid is at or above room temperature and significantly belowthe liquid boiling point, T_(B).

As the microwave heats the liquid ammonia in the penetration depth 3,the ammonia begins to evaporate. The purified gaseous ammonia 10, isdrawn off the vaporizer system. As a result of withdrawing vapor, thevapor pressure of the liquid ammonia begins to drop as does thetemperature of the liquid in the penetration depth 3. To prevent spotboiling, microwave power is added to ensure that the liquid temperaturein the penetration depth does not drop to a point where the liquidstarts to boil at the reduced pressure. Point 3′ on FIG. 3 illustrateswhere the ammonia would be in the event that the pressure drops tooquickly. In that case, the ammonia would be too far below the agitationcurve and impurities would be allowed to enter the gas product.

The vapor pressure—temperature equilibrium should be maintained suchthat the system operates at a temperature near the boiling temperaturefor the vapor pressure that the system operates at. In other words, thepower must maintain the system such that it operates near thevapor-liquid equilibrium line. See FIG. 3. This type of operationensures high enough flow rates without compromising purity.

The primary objectives of this invention are high product flow ratesalong with high purity. It is known that operating a vaporizer systemwith high superheat above T_(B) will give a large flow throughput, alongwith an increase in the impurity level. If we can accept this increasein impurity, then it is obvious to operate above T_(B) so as to achievethe high flow through put. This invention is nonobvious because itoperates near T_(B). In this invention, the superheat of liquid ammoniain the penetration depth is maintained at a temperature of less thanabout 30° F., preferably less than about 20° F., and most preferablyless than about 10° F.

FIG. 2 illustrates the concept of flow and impurity as a function ofoperating temperature. At T_(min) the impurity level of the vapor systemis very low. However, the corresponding flow rate of the vapor stream istoo low to meet the objectives of the invention, specifically, high flowrates of vapor products.

As the operating temperature increases, the flow rate starts approachingacceptable design levels. Above T_(D), the design temperature of thefluid in the penetration depth, the impurity levels could increase. AsFIG. 2 suggests, T_(D) is the optimal operating temperature. At T_(A)the flow rates are sufficiently high. Although the impurity level atthis temperature is projected to be higher than at lower temperatures,the impurity level still meets the objectives of the invention. AboveT_(C), the purity level is compromised and not acceptable for thesemiconductor industry.

Overall, FIG. 2 illustrates the appropriate trade-off between flow rateand impurity levels. At T_(D), the system provides acceptable flow ratesand impurity levels for the semiconductor industry.

To maintain the desired flow rate of evaporation, power must be added tothe system to replace the heat required for vaporization. If this is notdone, the temperature of the system will drop, and the rate ofevaporation will decrease, and the desired vapor flow will not beachieved. If the vapor draw is still maintained at the high desiredlevel, the vapor pressure will fall and the liquid will begin to boil.As a result, all purification advantage for moisture, oils and NVR willbe lost.

A standard evaporator approach using internal or external heaters withagitation tends to promote boiling by generating gradients andnucleation sites. Agitation also increases the mobility of NVR, therebyincreasing the chances of passage into the vapor phase.

The current invention uses microwave heating at a controlled power tosustain the evaporation process. An effective amount of microwave poweris added to the liquid so that the temperature of the liquid in thepenetration depth substantially remains near the boiling temperature ofammonia at the design vapor pressure.

FIG. 3 illustrates the desired operating conditions for the vaporizersystem. The system will be designed to operate within the designpressure and temperature parameters, P_(D) and T_(D), respectively. Notethat T_(D) is near the boiling temperature, T_(B), which corresponds tothe saturated pressure, P_(D). A cycle of the operating conditions willnow be discussed. At the beginning of the cycle, the system is at aboutroom temperature (T₃) and at atmospheric pressure (Patm). This isillustrated by point 1, FIG. 3. As microwave power is added, thepenetration depth is heated. The penetration depth is the top layer (upto 30 mm) of the liquid that is actually heated by the addition ofmicrowave energy. Also, as microwave power is added, the temperature andpressure of the penetration depth increases. In this way the designtemperature, T_(D), and the design pressure, P_(D), are reached. This isillustrated as point 3 on the diagram. Upon reaching the design pressureand temperature, the liquid evaporation rate begins to increase. At thebottom of the penetration depth, the temperature is slightly belowT_(D). The temperature here corresponds to T₂, from FIG. 1. The vaporleaves the liquid at the freeboard, which is the interface between thevapor and the liquid. At a higher vapor demand rate, the vapor flows outof the system at a faster rate, resulting in a decrease in the vaporpressure of the system. A large decrease in vapor pressure could causethe liquid to become too agitated at the design temperature of T_(D). Toprevent this, microwave power is increased, which raises the liquidtemperature above T_(D), resulting in increased evaporation of the fluidas desired.

Another result of the increased vapor flow, without increasing themicrowave power, is a concurrent temperature drop in the penetrationdepth. As the liquid turns to vapor, the liquid left behind is subcooledbelow T_(D), resulting in a decrease in flow. This is numeral 4 on FIG.3. To prevent the temperature and pressure from going below the minimumsof P_(L) and T_(L), microwave power is increased. This raises the liquidtemperature and vapor pressure to T_(D) and P_(D) as indicated bynumeral 3 on FIG. 3. The microwave power is able to restore thetemperature in the penetration depth. As a result, the vaporpressure-temperature equilibrium is restored and the cycle starts over.Overall, the invention maintains the pressure and temperature below thevapor-liquid equilibrium curve, but tries to maintain the pressure andtemperature below the agitation curve having R_(D) around 1.0″.

FIG. 4 illustrates the overall microwave vaporizer scheme for thepresent invention. Cylinder 1 supplies the evaporator 10 with impurefeed ammonia via line 7. First, feed stream 2 goes through an ESO(emergency shut off) panel 3, resulting in stream 4. As long as valve 15is open, feed continues as stream 5 through valve 6, and enters theevaporator 10 as stream 7. The cylinder feeds to evaporator 10 to a setlevel.

Line 11 transmits the level or weight transmitter 12 of the specificlevel of ammonia. This information is sent to the level controller 13,which controls (via line 14) valve 15 to modulate the ammonia feed flow.If the ammonia level in the tank is too low, the valve opens allowingammonia liquid feed to fill the tank. If there is enough ammonia, thevalve closes.

In the event that the system needs to be purged, nitrogen tank 20 feedspurge gas panel 22 via line 21. The nitrogen is then fed to ESO panel 3via purge line 23. Purging with nitrogen and inerts ensures that thevaporizer system is dry.

Once the evaporator is suitably filled, the microwave power is suppliedby turning on magnetron 30. The microwave power supply and magnetron 30is turned on when the product ammonia flow is required. The microwaveunit is capable of supplying a high number of watts and its duty cycleis controlled by the combination of a reverse acting pressure controller58 and reverse acting temperature controller 53 via a line 61. Thepressure and temperature controllers are located inside of the ammoniatank, 81, 82, 83, and 84. The microwave power supply and magnetronoperate at 2.45 Ghz or any suitable frequency in the microwave region ofthe electromagnetic spectrum.

Single-mode microwaves generated by magnetron 30 conducted down waveguide 31 are introduced into the evaporator by way of a speciallydesigned safety docking collar 32.

The incident wave, from the microwave, that approaches the planevapor-liquid interface between the two phases result in a transmittedwave in the liquid media and a reflected wave in a vapor media. Ammoniais a dielectric; therefore, the electric field and magnetic field depthof penetration into liquid ammonia will be limited. Under the abovedescriptions of operation the penetration depth is from about 16 mm toabout 20 mm below the freeboard surface at a temperature of about 20° C.Upon heating, about 16 mm to about 20 mm layers of ammonia below thefreeboard will evaporate. Product vapor is withdrawn through floworifice 42 and flow limit control valve 47, which is controlled byreverse acting flow controller 45. The current invention provides theproduct flow to customer battery limits at a rate of up to 2,500 gaseouscc/min. The average power input is between 200 and 4,500 Watts/ft². Inthe case of ammonia, the microwave energy flux used is from about 0.3 toabout 10 watt/in², preferably from about 0.3 to about 20 watt/in² , andmost preferably from about 0.3 to about 30 watt/in². This isaccomplished by maintaining the thermal equilibrium between the liquidvapor phase. The PID (proportional integral derivative) control loopswhich controls the microwave power can rapidly respond to the need formaintaining steady vapor pressure by replenishing heat to the system,lost by the high flow rate of the product.

Gaseous ammonia flows from the ammonia tank. Line 40 flows through floworifice 42, which flows through flow limit control valve to produceultra-high purity gaseous ammonia product stream 48. At the same time,flow transmitter 43 measures the flow through flow orifice 42. Thetransmitter informs the flow controller of the flow level. Flowtransmitter information is passed onto flow controller 45, whichinstructs the flow limit control valve to open or close.

Information regarding temperature and pressure are read at the top ofthe ammonia tank. Temperature information is transmitted to temperaturetransmitter via line 51. Meanwhile, pressure information is transmittedto pressure transmitter via line 56. Although displayed externally, thetemperature transmitters and pressure transmitters are actually locatedinside of the system. The transmitters are located above the freeboard.The temperature transmitter and pressure transmitter transduce theinformation into a signal recognizable by the temperature controller,53, and pressure controller, 58. Temperature and pressure controllers 53and 58 feed their information to low select, 60, which instructs (vialine 61) the magnetron how much microwave power is required to restoreequilibrium. With increased product flow, the vapor pressure, P_(V),falls which, in turn causes an increase in the microwave power.Consequently, as the temperature, T_(PD), in the penetration depthincreases, the microwave power is reduced, which in turn reduces thevapor pressure and the cycle of power control continues.

The pressure controller, temperature controller and low select comprisea universal controller. The low select, 60, determines whether themagnetron, 30, should be on or off. Low select 60 operates by selectingthe lowest relative value between temperature and pressure. Toillustrate, if the pressure level is relatively lower than thetemperature level, then the low select, 60, will “low-select” thepressure level and disregard the temperature inputs (as long as thetemperature is not too high). Low select 60 will recognize that pressureis falling and then the magnetron will be instructed to turn on, vialine 61. In the event that the pressure is the low selected signal, butis not below design parameters, line 61 will instruct the magnetron tostay off. The above description will also work in the event thattemperature was the low-selected signal.

As the amount of liquid in the tank decreases due to evaporation theimpurities in the bulk liquid increases. When the impurity levels becometoo high, the waste liquid is removed from the tank. Ammonia is drawnoff through line 70, through valve 71, forming stream 72, which isstored in waste cylinder 73.

This invention is operable in the operational at a pressure betweenabout 115 to about 120 psig, and at a temperature of between about 18°C. to about 21° C.

This invention may be operated using any polar saturated liquids,however, for purposes of this invention, particularly interestedsaturated liquid may include, but are not limited by NH₃, HF, SiHCl₃,SiH₂Cl₂, C₄H₈, C₃F₈, HBr, C₅Fe, ClF₃, TEOS and the like. The frequencychosen for this device is 915, 2450, 5850, and 18000 MHz. Flow ratesfrom 1 to 2,500 l/min or higher can be achieved by this process withmoisture and NVR levels less than about 1 ppm. Contaminants that remainin the liquid pool can be continuously dumped with continuous filling orperiodic dump. The system's performance is optimized by integrating ageometric shape with multiple magnetrons located surrounding liquidvolume on a specially designed shape. This may reduce the number ofmicrowave interference pattern than can create spot focusing and unevenheating.

Specific features of the invention are shown in one or more of thedrawings for convenience only, as each feature may be combined withother features in accordance with the invention. Alternative embodimentswill be recognized by those skilled in the art and are intended to beincluded within the scope of the claims.

1. A method for separating a polar ammonia liquid comprising a)introducing an effective amount of microwave energy at a depth rangingfrom 2.6 mm to 30 mm into a top layer of the liquid; b) controlling themicrowave energy that is introduced into the top layer of the liquid tomaintain a temperature below the boiling point of the top layer ofliquid; c) evaporating the top layer of liquid in the presence of themicrowave energy while sustaining the temperature below the boilingpoint of the liquid to form purified vapors; and d) capturing theevaporated purified vapors.
 2. The method of claim 1 comprisingcontrolling the vapor pressure of the liquid.
 3. The method of claim 1further comprising applying microwave energy at a frequency between 915MHz to 18000 MHz.
 4. The method of claim 1 maintaining the microwaveenergy that is introduced into the liquid to evaporate the liquid at aflow rate between 1 to 1000 1/min.
 5. A method for separating a polarammonia liquid comprising a) introducing microwave energy at a depth ofranging from 2.6 mm to 30 mm into a top layer of the liquid to reach thetop layer's boiling point; b) controlling the microwave energy to nearthe boiling point of the top layer; c) evaporating the top layer ofliquid in the presence of the microwave energy while sustaining thetemperature below the boiling point of the bulk liquid disposedunderneath the top layer of liquid to form purified vapors; and d)capturing the evaporated purified vapors.
 6. The method of claim 5comprising controlling the vapor pressure of the liquid.
 7. The methodof claim 5 further comprising applying microwave energy at a frequencybetween 915 MHz to 18000 MHz.
 8. The method of claim 5 comprisingmaintaining the microwave energy that is introduced into the liquid toevaporate the liquid at a flow rate between 1 to 1000 1/min.